Hard drive would just quit working, tried 2 new hard drives, still the same problem. So if it is not a hard drive problem, and the logic board worked fine with an external drive, then it must be the hard drive connector cable.
Ordered the correct cable from iFixit, replaced the old cable, and presto! no more problem...Yea!
Diagnosing the problem as a cable problem is the harder part of the task. Replacing the hard drive cable, on this model MacBook Pro was very easy...